Secretary of State John Kerry has intervened to delay the execution of a convicted Mexican saying his rights may have been violated, in a case that could endanger Americans abroad, a US official said.

Edgar Arias Tamayo was found guilty of the 1994 fatal shooting of a Houston police officer, and is set to be put death by the state of Texas next month.

But Kerry has warned that he was not given his right to see Mexican consular officials as the United States is obligated to do under an international convention.

In a letter, Kerry urged Texas Governor Rick Perry to delay the execution until a judicial review of Tamayo's case is completed.

Kerry "doesn't have any reason to doubt the facts of the conviction," State Department spokeswoman Marie Harf told reporters, adding that, as a former prosecutor, the top US diplomat has "no sympathy for anyone who would murder police officers."

But it was clear that Tamayo had not been afforded consular access as laid out in what is known as the Vienna Convention.

Ensuring that the United States complies with international law "is critical to our ability to ensure consular access and protection for our own citizens," Harf stressed.

"How can we go around the world and ask other countries to give our folks consular access if they are in imprisoned (and) if we don't do the same thing here?" she asked.

Kerry has urged the governor to stay the execution until it can be determined whether lack of consular access "prejudiced the outcome of the case."

"This issue is particularly important to our bilateral relationship with Mexico," Kerry wrote in the September 16 letter, a copy of which was sent to AFP.

"If an execution date is set... it would unquestionably damage these vital US interests," he added.

Tamayo is, in fact, one of 51 Mexican nationals named in the so-called 2004 Avena case, in which the UN's International Court of Justice ordered the US to review whether their sentences and convictions were prejudiced by the violation of the Vienna Convention.

Texas has yet to comply with the ICJ order.

Mexico's ambassador to the US, Eduardo Medina Mora, wrote to Kerry in August warning the issue has become "a significant irritant in the relations between our two countries."